This application is based on and claims the priority date of German Application No. 101 48 403.8, filed on Sep. 29, 2001, which is incorporated herein by reference.
Electromagnetic valve drives for actuating the cylinder valves in a piston-type internal-combustion engine essentially comprise two electromagnets, which are spaced from one another, and an armature, which is connected to the cylinder valve to be actuated and is guided back and forth, counter to the force of restoring springs, between the magnet pole faces that face one another. The coils of the electromagnets are alternately acted upon with a current via a control device at predeterminable intervals, corresponding to the operating data of the piston-type internal-combustion engine, primarily the rpm and, accordingly, the work cycle. After the current is cut off, the force of a restoring spring moves the armature resting against the pole face of one electromagnet in the direction of the pole face of the other electromagnet. As the armature approaches, the respective other magnet is acted upon by a (high) capturing current, so the armature comes to rest against its pole face due to the capturing magnetic field, and counter to the force of a restoring spring associated with the capturing electromagnet. With a corresponding control of the current supply, the high capturing current can be reduced to a retaining current immediately before or upon the impact of the armature against the pole face, so the affected cylinder valve can accordingly be held in the open or closed position with a low current consumption.
The armature should impact the pole face of the respective capturing magnet at the lowest possible speed to avoid bouncing. Thus, the kinetic energy inherent to the armature is of great significance in the equilibrium phase preset by the opposing restoring springs. Because the spring energy of the tensed restoring spring that acts on the armature is practically constant at the respective retaining magnet, it is only possible to influence the maximum kinetic energy during the armature""s passage through the equilibrium position by altering the current supply to the capturing or the retaining magnet.
At the capturing electromagnet, the current supply is controlled such that the armature moves in a linear ramp or slope for the last tenths of a millimeter prior to impacting the pole face. For this purpose, it is necessary that the armature not swing through, i.e., have an excessively high kinetic energy, during this final approach phase. With a high engine load, this is attained through a xe2x80x9cconsumptionxe2x80x9d of kinetic energy due to the counterpressure in the cylinder combustion chamber, which supports the current control. With a low load, however, especially in idling operation, the effect of the counterpressure is so weak that practically no xe2x80x9cbrakingxe2x80x9d effect, that is, no xe2x80x9cconsumptionxe2x80x9d of kinetic energy, occurs.
In the past, to achieve a braking effect with a low-load, a braking current pulse was applied to the retaining electromagnet immediately following the cutoff of current when the armature began to move. Consequently, a magnetic force that counteracted the force of the restoring spring briefly acted on the armature, thus xe2x80x9csappingxe2x80x9d a corresponding quantity of kinetic energy at the start of the armature movement.
Controlling idling operation with a braking current pulse also has its drawbacks. In particular, the capturing current at the capturing magnet and the braking current at the releasing magnet overlap, the valve flight time is increased, and the maximum speed is lower during the passage through the equilibrium position. To compensate for this, the spring-mass oscillator comprising the armature, cylinder valve and restoring springs would have to be faster, i.e., stronger restoring springs would have to be used. A further drawback, which is specific to a rapid change in load, for example, from idling in the direction of full-load operation, is that the control device would already have to decide prior to the armature movement whether the braking current pulse should be activated. This precludes the option of utilizing sensor information about the actual armature movement, which can be obtained with modern sensor equipment.
It is the object of the invention to provide a method that can avoid an operating mode requiring braking current pulses.
According to the invention, the above object generally is achieved by a method for controlling an electromagnetic actuator for actuating a cylinder valve in a piston-type internal-combustion engine, with the valve having two electromagnets that are spaced from one another, and an armature that is connected to the cylinder valve to be actuated and is guided back and forth, counter to the force of at least one restoring spring, between facing magnet pole faces of the two electromagnets, and a control device for controlling the current supplied to the coils of the electromagnets. This method comprises: using the control device, alternately supplying the coils of the electromagnets with a direct current at predetermined intervals; determining whether the piston-type internal-combustion engine is operated under a high load or under low-load operation (idling mode); and, controlling the flow direction of the current through the coils of the two electromagnets so that (a) for high-load operation, the magnetic flux alternately effected in the armature by the respective currents in the two coils is unidirectional, and (b) for low-load operation (idling mode), the magnetic flux alternately effected in the armature by the respective currents in the two coils flows in opposite directions due to repolarization of the flow direction of the current through one of the two coils.
The advantage of the invention is that, when the piston-type internal-combustion engine is operating under a high load, the control device presets the flow direction of the current through the coils of the two electromagnets such that the magnetic flux in the armature, which is effected alternately by the two coils, remains unidirectional. This eliminates a remagnetization, so the residual magnetism remaining in the still-moving armature after the retaining current has been cut off prevents magnetic hysteresis losses. The armature, and thus the cylinder valve to be actuated, are moved back and forth without a time delay, which assures reliable operation, even with a large load, and especially at high rpms.
For low-load operation, particularly for idling operation, the polarity of the current of one coil is reversed, which changes the flow direction of the current through the coil for the duration of this load, so that the magnetic flux effected or produced in the armature alternately flows in opposite directions when the coil of the respective capturing magnet is supplied with current. In other words, the armature is alternately remagnetized. As a consequence of this remagnetization, when the current increases, the introduced electrical energy is not converted into kinetic energy of the armature, but into magnetic hysteresis losses and, finally, thermal losses. A positive effect of this former practice of xe2x80x9cfalselyxe2x80x9d polarizing the coils of an electromagnetic valve drive is that, in idling operation, less kinetic energy can be coupled in without costly switching and control measures, and without adversely affecting the armature flight time. For both full-load and idling operation, the current can be set at a high level. Nevertheless, for idling operation, only a reduced quantity of kinetic energy is supplied to the armature, because a portion of the electrically fed-in energy is expended through magnetic hysteresis losses. When the operating mode is switched from idling to a higher load, it is only necessary to reverse the polarity of the current in order to introduce the electrical energy into the system with a high efficiency.
In comparison to solutions in which the current is set at the desired level for supplying the kinetic energy required by the armature for the respective load, the method of the invention is less costly. Particularly in comparison to control systems having a boost voltage or a generally high voltage level, the method of the invention requires a lower outlay for components.
The invention is described in detail by way of schematic drawings.